TagBot
Originally downloaded March 18, 2007. TagBot is the third application I know of that aims to make tagging and smart folders as much of a reality as Apple and many visionaries hope they will. The other two are freeware (SpotMeta and Punakea), but TagBot will cost you $20 if you need more than 6 tags. I haven't yet looked to see how TagBot works, but I'm downloading it because I think this has the potential to be a vital improvement over mere folders for organizing files.
Version as tested: 1.0.1.
Update 9/5/12. This software is no longer in development or supported.